Impaired insulin secretion is a key determinant of the onset of diabetes. In contrast to insulin resistance, which is predominantly due to increased lipid supply to muscle and liver from expanded intra-abdominal adipose tissue depots, defects in insulin secretion appear to be largely genetically-determined. Estimation of glucose-induced insulin secretion is challenging due to complex relationships with insulin sensitivity and hepatic insulin clearance. In the basal state, HOMA-B can be derived from fasting glucose and insulin. The use of C-peptide, rather than insulin, has been proposed, in order to minimise confounding relating to hepatic insulin extraction. The insulin response to intravenous glucose injection (intravenous glucose tolerance test [IVGTT]) is a common method used to measure stimulated insulin secretion. First-phase insulin secretion (acute insulin response [AIR]), due to the release of preformed β-cell insulin granules, is calculated as the average of insulin or C-peptide levels, or the area under the curve, 0-10 min after glucose administration. In order to ‘correct’ for prevailing insulin sensitivity, the product of AIR and insulin sensitivity (derived from the IVGTT by minimal modelling), termed the ‘disposition index’ (DI), is calculated. The IVGTT can be followed by a hyperglycaemic clamp, with second-phase insulin secretion calculated from insulin levels measured during the last 30 min of the clamp. Alternatives to the IVGTT include the oral glucose tolerance test or mixed meal, which incorporate the incretin response to glucose and amino acids, possibly making these techniques more physiological. The protocol, which uses C-peptide levels following oral glucose/meal ingestion, can also be used to measure insulin sensitivity, allowing calculation of the oral DI. In summary, basal and stimulated measurements are used to estimate insulin secretion in the research setting. Stimulated methods allow the simultaneous measurement of insulin sensitivity, thus allowing insulin secretion to be interpreted in light of prevailing insulin sensitivity.